Mobile communications system refers generally to any telecommunications system which enable wireless communication when users are moving within the service area of the system. A typical mobile communications system is a Public Land Mobile Network (PLMN). Often the mobile communications network is an access network providing a user with wireless access to external networks, hosts, or services offered by specific service providers.
The general packet radio service GPRS is a new service in the GSM system (Global System for Mobile communication). A subnetwork comprises a number of packet data service nodes SN, which in this application will be referred to as serving GPRS support nodes SGSN. Each SGSN is connected to the GSM mobile communication network (typically to a base station controller BSC or a base station BTS in a base station system) so that the SGSN can provide a packet service for mobile data terminals via several base stations, i.e. cells. The intermediate mobile communication network provides radio access and packet-switched data transmission between the SGSN and mobile data terminals. Different subnetworks are in turn connected to an external data network, e.g. to a public switched data network PSPDN, via GPRS gateway support nodes GGSN. The GPRS service thus allows to provide packet data transmission between mobile data terminals and external data networks when the GSM network functions as a radio access network RAN.
Third generation mobile systems, such as Universal Mobile Communications system (UMTS) and Future Public Land Mobile Telecommunications system (FPLMTS), later renamed as IMT-2000 (International Mobile Telecommunication 2000), are being developed. In the UMTS architecture a UMTS terrestrial radio access network, UTRAN, consists of a set of radio access networks RAN (also called radio network subsystem RNS) connected to the core network (CN). Each RAN is responsible for the resources of its set of cells. For each connection between a mobile station MS and the UTRAN, one RAN is a serving RAN. A RAN consists of a radio network controller RNC and a multiplicity of base stations BS. One core network which will use the UMTS radio access network is the GPRS.
One of the main targets in the development of mobile communication networks is to provide an IP (Internet Protocol) service with a standard IP backbone which would use a combination of different types of mobile network mobility managements in the mobile networks and Mobile IP. The basic IP concept does not support the mobility of the user: the IP addresses are assigned to network interfaces in dependence on their physical location. In fact, the first field of an IP address (the NETID) is common to all interfaces that are linked to the same Internet subnet. This scheme prevents the user (the mobile host) from keeping its address while moving over different Internet subnets, i.e. while changing the physical interface.
In order to enhance the mobility in the Internet, a Mobile IP protocol for IP version 4 have been introduced by the Internet Engineering Task Force (IETF) in the standard RFC2002.Mobile IP enables the routing of IP datagrams to mobile hosts, independently of the point of attachment in the subnetwork. The mobile IP protocol introduces the following new functional or architectural entities.
‘Mobile Node MN’ (also called Mobile Host MH) refers to a host that changes its point of attachment from one network or subnetwork to another. A mobile node may change its location without changing its IP address; it may continue to communicate with other Internet nodes at any location using its (constant) IP address. ‘Mobile Station (MS)’ is a mobile node having a radio interface to the network. A ‘Tunnel’ is the path followed by a datagram when it is encapsulated. In this model a datagram, while encapsulated, is routed to a known decapsulation agent, which decapsulates the datagram and then correctly delivers it to its ultimate destination. Each mobile node is connected to a home agent over a unique tunnel, identified by a tunnel identifier which is unique to a given Foreign Agent/Home Agent pair.
‘Home Network’ is the IP network to which a user logically belongs. Physically, it can be e.g. a local area network (LAN) connected via a router to the Internet. ‘Home Address’ is an address that is assigned to a mobile node for an extended period of time. It may remain unchanged regardless of where the MN is attached to the Internet. Alternatively, it could be assigned from a pool of addresses.
‘Mobility Agent’ is either a home agent or a foreign agent. ‘Home Agent HA’ is a routing entity in a mobile node's home network which tunnels packets for delivery to the mobile node when it is away from home, and maintains current location information for the mobile node. It tunnels datagrams for delivery to, and, optionally, detunnels datagrams from, a mobile node when the mobile node is away from home. ‘Foreign Agent FA’ refers to a routing entity in a mobile node's visited network which provides routing services for the mobile node while registered, thus allowing the mobile node to utilise its home network address. The foreign agent detunnels and delivers packets to the mobile node that were tunnelled by the mobile node's home agent. For datagrams sent by a mobile node, the foreign agent may serve as a default router for registered mobile nodes.
RFC2002 defines ‘Care-of Address’ (COA) as the termination point of a tunnel toward a mobile node for datagrams forwarded to the mobile node while it is away from home. The protocol can use two different types of care-of address: a “foreign agent care-of address” is an address announced by a foreign agent with which the mobile node is registered, and a “co-located care-of address” is an externally obtained local address which the mobile node has acquired in the network. An MN may have several COAs at the same time. An MN's COA is registered with its HA. The list of COAs is updated when the mobile node receives advertisements from foreign agents. If an advertisement expires, its entry or entries should be deleted from the list. One foreign agent can provide more than one COA in its advertisements. ‘Mobility Binding’ is the association of a home address with a care-of address, along with the remaining lifetime of that association. An MN registers its COA with its HA by sending a Registration Request. The HA replies with a Registration Reply and retains a binding for the MN.
A single generic mobility handling mechanism that allows roaming between all types of access networks would allow the user to conveniently move between fixed and mobile networks, between public and private networks as well as between PLMN's with different access technologies. Therefore, mechanisms supporting the Mobile IP functionality are also being developed in mobile communication systems, such as UMTS and GPRS.
It is desired that the Mobile IP be implemented as an overlay of the UMTS/GPRS network while maintaining backwards compatibility with present systems, assuming minimal modifications in the GPRS standards and in networks whose operators do not want to support the MIP. FIG. 1 illustrates the minimum configuration for a GPRS operator who wishes to offer the mobile IP service. The current GPRS structure is maintained, i.e. the GPRS handles the mobility within the PLMN, while the MIP allows the user to roam between other systems, such as LAN's, and UMTS without loosing an ongoing session. In FIG. 1 the foreign agents FA are located at GGSNs. All GGSNs may not have FAs. The SGSN and the GGSN may also be co-located. One FA in a PLMN is sufficient for offering the MIP service, but for capacity and efficiency reasons, more than one may be recommendable. This means that the MS must request a PDP context to be set up with a GGSN that offers FA functionality. While setting up the PDP context, the MS is informed about network parameters of the FA, e.g. care-of address.
The MS may have the same care-of address COA during a session, i.e. as long as a PDP context is activated. A very mobile MS might perform several inter-SGSN HOs during a long session which may cause inefficient routing. As an initial improvement, a streamlining procedure, with a temporary anchoring point in the GGSN, could be introduced: If the MN is not transferring data, or is possibly even in the active state while moving from one SGSN to another, a new PDP context could be setup between the new SGSN and its associated GGSN at the handover. The MN will receive a new care-of address. If the MN were transferring data, e.g. were involved in a TCP session, the MN would move from the old SGSN to the new one while keeping the PDP Context in the old (anchor) GGSN for the duration of the data transfer. Once the data transfer is terminated, the PDP Context can be moved to the GGSN associated with the new SGSN and a new care-of address can be obtained.
The problem is how to discover the movement and to find a new foreign agent FA, preferably the nearest one, when the MN is moving from one SGSN to another. The GPRS terminal (MS) is naturally aware of the change of the SGSN on the GPRS protocol level as described above but this change is transparent to the overlaying MIP protocol and the mobile node MN associated with the GPRS terminal MS.
Similar problems may be encountered in any mobility management and routing on a system level overlaying the mobility management of an access network. These various overlaying mobility managements are commonly referred to as macro mobility management herein.